1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device for comparing current image data with past image data, generating corrected data, and driving liquid crystals by means of the corrected data.
2. Description of the Related Art
In a standard active matrix type liquid crystal display device, the scanning period for one screen image (one frame) is between approximately 50 Hz and 75 Hz. On the other hand, the optical response of liquid crystal molecules requires several 10 ms. Therefore, if a moving image, such as a TV image, is displayed on a liquid crystal display device, then the liquid crystal response cannot follow the changes in the display data of the liquid crystal display device, thereby leading to the problem of latent images.
One of the conventional methods implemented in order to resolve the problem of latent images of this kind is methods which concentrates on the dependence of the response speed of the liquid crystal molecules on the voltage applied thereto. FIG. 16 shows a schematic diagram of the relationship between the liquid crystal applied voltage and liquid crystal response (luminosity change). The diagram shows the case of a liquid crystal display device in “normally white mode” which provides a white display when no voltage is applied. The vertical axis of the graph shows liquid crystal applied voltage and luminosity, and the horizontal axis shows time. In this example, the luminosity change when the liquid crystal applied voltage change is Vx is taken as Bx, and the luminosity change when the liquid crystal voltage change is Vy is taken as By. Furthermore, before timing t1, the previous image data, being the image data for the previous frame, is indicated, and after timing t1, the current image data, being the image data that is currently to be displayed, is indicated.
In this diagram, if the change in the liquid crystal applied voltage is Vx, then in accordance with the change in voltage from the voltage corresponding to the previous image data to the voltage corresponding to the current image data, on either side of timing t1, the luminosity change Bx reaches a prescribed luminosity at timing t2. If, on the other hand, the change in liquid crystal applied voltage is Vy, then in accordance with the change in voltage from the voltage corresponding to the previous image data to the voltage corresponding to the current image data, at timing t1, the luminosity change By reaches a prescribed luminosity at timing t3. As shown in the diagrams, the time period from timing t1 to timing t3 is longer than the time period from timing t1 to timing t2, and hence the larger change in the liquid crystal applied voltage, Vx, causes the prescribed luminosity to be reached more quickly than the voltage change Vy. Accordingly, it can be seen that the time period from the start of response by the liquid crystal until completion of that response, induced by a change in the liquid crystal applied voltage, is quicker, the greater the amount of change in the liquid crystal applied voltage. In other words, the liquid crystal response between black and white is faster than the liquid crystal response between intermediate tones.
Next, a method for improving the response of liquid crystals between intermediate tones is described. FIG. 17 is a schematic diagram of the relationship between the liquid crystal applied voltage and the liquid crystal response (luminosity change). As shown in this diagram, when changing from a dark intermediate tone to a lighter intermediate tone, a lower voltage than the steady electric potential after the change is applied temporarily, thereby speeding up the optical response of the liquid crystals. In this example, the normal change in liquid crystal applied voltage is taken as Vy, and the change in liquid crystal applied voltage according to this improvement method is taken as Vz. The luminosity change in the case of a liquid crystal applied voltage change of Vy is taken as By, and the luminosity change in the case of a liquid crystal applied voltage change of Vz is taken as Bz. Moreover, the period before timing t1 indicates previous image data and the period after timing t1 indicates current image data.
In the diagram, if the change in the liquid crystal applied voltage is Vy, then in accordance with a voltage change at timing t1 from the voltage corresponding to the previous image data to the voltage corresponding to the current image data, the luminosity change By reaches a prescribed luminosity at timing t31. If, on the other hand, the change in the liquid crystal applied voltage is Vz, then in accordance with a voltage change at timing t1 from the voltage corresponding to the previous image data to the voltage corresponding to the current image data, the luminosity change Bz reaches the prescribed luminosity at timing t32. As shown in the diagram, the time period from timing t1 until timing t32 is shorter than the time period from timing t1 to timing t31, and hence a prescribed luminosity can be reached more quickly by adopting the voltage application method according to this improvement method.
If changing from a lighter intermediate tone to a darker intermediate tone, then the optical response of the liquid crystal is speeded up by temporarily applying a higher voltage than the steady electric potential after change. By correcting the liquid crystal applied voltage in this way, it is possible to improve the liquid crystal response characteristics between intermediate tones.
Japanese Patent No. 2,616,652 discloses a liquid crystal drive method and liquid crystal display device whereby, in order to correct the liquid crystal applied voltage corresponding to the current image data, from the relationship between the current image data and the image data for the previous frame, in this aforementioned manner, the data for the previous frame is stored, and the liquid crystal applied voltage is determined by comparing this stored data with the current image data. A concrete composition of a liquid crystal display device applying a method for improving liquid crystal response between intermediate tones, as disclosed in the aforementioned patent, is described below with reference to the drawings. In the example in FIG. 18, the resolution is XGA (1024×3×768), and only the portion of the 256-colour display liquid crystal display device which relates to signal processing is illustrated. In FIG. 18, 1 denotes a timing controller, 2 denotes a frame memory for inputting and storing image data 12 from the timing controller, and 3 denotes data comparing and corrected data generating means, for inputting the current image data 14 from the timing controller 1, inputting the previous image data 13 from the frame memory 2, comparing the two sets of data, and generating corrected data. 4 denotes a signal line driving circuit for driving the signal lines of a liquid crystal panel 6, on the basis of the corrected data and a control signal 16 output by the data comparing and corrected data generating means 3. 5 denotes a scanning line driving circuit for driving scanning lines of the liquid crystal panel 6 on the basis of a control signal 17. 6 denotes a liquid crystal panel, being an active-matrix type liquid crystal panel, such as a TFT (Thin Film Transistor) liquid crystal panel, or the like.
Next, the operation of the device is described. Signals 11, such as a clock signal (CLK), a horizontal synchronization signal (HD), a vertical synchronization signal (VD), a data interval normalizing signal (DENA), a data signal (RGB DATA) input to the liquid crystal display, are input to the timing controller 1. Image data 12 consisting of 8-bit RGB data respectively, is input from the timing controller 1 to the frame memory 2. The image data (previous image data) used to display the previous frame, as input from the timing controller 1, is stored in the frame memory 2. The timing controller 1 outputs control signals 16, 17 for controlling the signal line drive circuit 4 and the scanning line drive circuit 5, to the respective drive circuits 4, 5, and it outputs the current image data 14 to the data comparing and corrected data generating means 3.
The data comparing and corrected data generating means 3 compares the current image data 14 input from the timing controller 1 with the previous image data 13 transferred from the frame memory 2, generates corrected data, and outputs same to the signal line driving circuit 4. Liquid crystal applied voltages corresponding to the corrected data 15 comprising respective 8-bit RGB data input by the signal line driving circuit 4 is supplied to the liquid crystal panel 6.
In this way, a frame memory for storing the previous image data for each picture element is required in order for the data comparing and corrected data generating means 3 to generate corrected data by comparing the previous image data 13 with the current image data 14. Moreover, in order to correct the liquid crystal applied voltages, in the data comparing and corrected data generating means 3, it is possible to adopt either a method whereby a look-up table is provided for reading out corrected data according to the relationship between the previous image data and the current image data, or a method whereby corrected data is determined by calculation from the relationship between the previous image data and the current image data. It is also possible for the data comparing and corrected data generating means 3 to be incorporated within the timing controller 1.
Further references disclosing prior technology include Japanese Patent Laid-open No. H5-183743, Japanese Patent Laid-open No. H5-336376, Japanese Patent Laid-open No. H10-143111, and Japanese Patent Laid-open No. H11-338424.